Vertcle Dirtribution And Significance Of Polycyclic Aromatic Hydrocarbons And Particle Size In Permafrost Of Da Hinggan Mountains

Polycyclic aromatic hydrocarbons（PAHs）are a kind of persistent organic pollutants commonly existing in the environment,which can enter the soil through a series of processes such as surface runoff and atmospheric deposition.PAHs will remain in the soil for a long time due to their indegradable properties in the environment,which poses a threat to the ecological environment and human health.Permafrost area of the Da Hinggan Mountains is located in the high latitude area of China.It is easy to deposit in the high latitude and low temperature area because of the global fractionation effect of PAHs.In addition,the pollution of PAHs in the remote high latitude area cannot be ignored under the influence of human activities and climate change.In the current study,a6.65 m long permafrost drill was used to collect a total of 38 soil samples in the vertical direction.The content,vertical distribution,sources,risk assessment of PAHs in permafrost and the relationship between PAHs and grain size were analyzed and its environmental significance were discussed by ¹⁴C radiometric dating.The main results were as follows:（1）The age of permafrost drill in Da Hinggan Mountains was about 15480±40–2410±30a BP,which indicated permafrost in the area has been formed since late Pleistocene.The total content of PAHs(Σ₁₆PAHs)in the sample ranged from 208.73 to 160.99 ng/g with the mean value of1334.96 ng/g.The concentrations of seven carcinogenic PAHs（Σ₇PAHs）including BaA,Chr,BbF,BkF,BaP,DahA and IcdP were in the range of 90.02–984.03 ng/g with an average value of 570.88ng/g.High molecular weight PAHs（4–6 ring PAHs）and low molecular weight PAHs（2–3 ring PAHs）accounted for 60.44%and 39.56%of the total PAHs,respectively.3–ring PAHs were the dominant compounds,and was significantly higher than other PAHs.The PAHs in the surface soil（0–10cm）of drill were compared with those in the permafrost of other regions and the soil near the study area.The surface of permafrost in Da Hinggan Mountains was of medium pollution level.（2）From the top to the bottom,the permafrost drill is divided into active layer,ice-rich layer of permafrost and peat-rich layer of permafrost.According to the vertical distribution characteristics of PAHs in the drill,the content ofΣ₁₆PAHs andΣ₇PAHs was the most in the ice-rich layer,and was significantly higher than that in the peat-rich layer layer（p<0.05）.The PAHs trapped in the ice-rich permafrost layer melt into the groundwater,causing secondary pollution when the climate is further warming.The concentrations of PAH increased gradually and reached maximum from surface to the bottom of active layer,which implyed the permafrost acts as a low-permeability barrier for migration of PAHs.The concentrations of PAH in ice-rich permafrost layer reached to a high peak after a short-term decline,and then decreased dramatically.It was speculated that there has been a warmer period in history.While in the peat-rich permafrost layer,the contents of PAH fluctuated greatly.（3）The sources of PAHs in permafrost drill were analyzed through factor analysis and ratio method with Fla/Pyr and Ba A/Chr.The results showed that volcanic activity,biological precursor diagenesis and ancient forest fire were the main sources of PAHs.The petroleum source was only identified in the active layer,which might be caused by the construction process of China-Russia oil pipeline.The contribution of local transportation sources has gradually increased due to the frequent occurrence of forest fires and human activities in recent years.（4）Risk assessment with ERL and ERM value showed that the ecological risk of total PAHs was relatively low in permafrost of the study area,while PAHs such as Acy,Ace,Flu and DahA have posed ecological threats to most of the samples.The results of BaP displayed that PAHs were at a safe level in the permafrost drill,indicating that there was no high health risk for the people in the study area.The results of incremental lifetime cancer risk assessment（ILCR）showed that the carcinogenic risk decreased in the order of skin contact>ingestion>respiration,the risk of carcinogenesis in adults was higher than that in children,and compared different layers of permafrost,It has been concluded that the relatively higher cancer risk in the ice-rich layer and lowest cancer risk in the active layer.（5）The ranges of particle size fractions were 0–1000μm in permafrost drill.2–20μm was the main particle size component,the content was in 33.46-74.25%,the content of>250μm were very low and the content range was 0–9.40%.The particle contents of<2μm and 2–20μm were contrary to those of 20–50μm,50–100μm and 100–250μm,and the dispersion degree of particle size distribution gradually increased from active layer to peat-rich layer of permafrost.The change trends of average and median particle size were consistent with those of>20μm components.According to the linear regression analysis of PAHs and depth in the active layer,it was found that the high molecular weight PAHs in the active layer has a selective downward migration,and there were significant positive correlation between 5–6-ring PAHs and 20–100μm particle size.（6）Combined with the analysis of the frequency distribution curve and the standard deviation curve of particle size,four environmentally sensitive components,C1（<2.19μm）,C2（2.19–19.95μm）,C3（19.95–181.97μm）and C4（181.97–831.76μm）were extracted from the active layer.Three sensitive components,C1（<4.37μm）,C2（4.37–39.81μm）and C3（39.81–208.93μm）were extracted from the ice-rich layer of permafrost.The three sensitive components in peat-rich layer of permafros twere C1（<3.33μm）,C2（3.33–22.91μm）,C3（22.91–239.88μm）.Pedogenesis was the main sensitive component in the whole drill,and a small amount of sources from atmospheric dust.There were very few coarse particles>200μm in the active layer,which represented the source of human activities.The 20–200μm component in active layer and ice-rich permafrost layer was the most sensitive to environmental change,indicating the component was caused by freezing and thawing.Contrast to the vertical contribution of the grain size sensitive components and the mechanical composition in the active layer,which showed that the process of freezing and thawing make the particles separate.The stepwise linear regression analysis of 5–6 ring PAHs and 20–100μm components implied that the selective downward migration of PAHs after the separation of particles caused by freezing and thawing would pollute the deeper soil.